Since the early 1980s, thousands of patients prone to irregular and sometimes life threatening heart rhythms have had miniature defibrillators and cardioverters implanted in their bodies. These devices detect onset of abnormal heart rhythms and automatically apply corrective electrical therapy, specifically one or more bursts of electric current, to hearts. When the bursts of electric current are properly sized and timed, they restore normal heart function without human intervention, sparing patients considerable discomfort and often saving their lives.
The typical defibrillator or cardioverter includes a set of electrical leads, which extend from a sealed housing through the veinous system into the inner walls of a heart after implantation. Within the housing are a battery for supplying power, a capacitor for delivering bursts of electric current through the leads to the heart, and monitoring circuitry for monitoring the heart and determining not only when and where to apply the current bursts but also their number and magnitude. The monitoring circuitry generally includes a microprocessor and a memory that stores instructions directing the microprocessor to interpret electrical signals that naturally occur in the heart as normal or abnormal rhythms. For abnormal rhythms, the instructions, or more generally signal-processing algorithm, tell the processor what, if any, electrical therapy should be given to restore normal heart function.
In general, these algorithms use the time intervals between successive heart beats, or cardiac events, as key determinants of therapy decisions. Thus, to ensure the validity of therapy decisions, it is very important to ensure accuracy of these intervals.
Determining these intervals can be especially problematic in dual-chamber defibrillation and cardioversion devices, which monitor the beats of two chambers of the heart, such as the left ventricle and the left atrium. In these devices, there is a significant risk of mistaking a ventricle beat for an atrial beat, and therefore counting too many atrial beats and miscalculating some atrial intervals (the time between atrial beats). Because of this risk, many dual-chamber devices use a technique, known as cross-chamber blanking, to ensure accuracy of atrial interval measurements.
Cross-chamber blanking entails using a blanking period to prevent sensing atrial beats for a certain time period after the last ventricular beat. In other words, atrial sensing is temporarily disabled after each ventricular beat to prevent mistaking the ventricular beat for an atrial beat. In conventional dual-chamber devices, the length, or duration, of the blanking period is fixed during manufacture and cannot be tailored to fit the unique needs of some patients. Accordingly, the inventors recognized a need for dual-chamber defibrillation and cardioversion devices that have programmable cross-chamber blanking periods.